This application is a 371 application of PCT/EP97/04678 filed Aug. 27, 1997.
1. Field of the Invention
The invention relates to a planar optical sensor platform, consisting of a transducer and a recognition layer, wherein changes in the effective refractive index of the recognition layer are converted into a measurable variable in accordance with the integrated-optical light pointer principle. The invention relates also to the use of the method and to the method itself using the sensor platform, for example in label-free biosensory analysis.
2. Description of the Related Art
The integrated-optical light pointer principle is described in WO 92/19976. It is based on the conversion of the change in the coupling angle, for coupling a light beam into a waveguide by means of a diffraction grating, into a position-dependent change in the signal, that is to say the coupling-in position, on the transducer. By means of the embodiments described below, the condition for coupling in a light beam covering a relatively large portion of the grating, which extends substantially perpendicularly to the direction of propagation of a guided mode, is met only at a specific position of the coupling-in grating. In the event of a change in the so-called effective refractive index for the guided mode, for example as a resuit of the adsorption of molecules on the grating, the coupling-in position is displaced on the grating perpendicularly to the direction of propagation of the guided mode. In this connection WO92/19976 proposes devices and methods wherein at least one coupling condition is made position-dependent. The position-dependency of the coupling condition is obtained preferably by varying the waveguide thickness or the grating period in a position-dependent manner or by varying other parameters, for example the refractive indices of substrate, cover layer and waveguide film, in a position-dependent manner. Combinations of the above position-dependent variations are also of particular interest. Special preference is given to two variants proposed in WO92/19976: with a coupling-in grating of constant period by the application of a waveguiding layer having a layer thickness that changes perpendicularly to the direction of propagation of the guided mode or by variation of the grating period of the coupling grating perpendicular to the mode propagation, with the layer thickness of the waveguide being constant. Further comments relating to the calculation, dimensioning and manufacture of transducers containing such integrated-optical light pointers can be found in xe2x80x9cReplicated chirped waveguide gratings for optical sensing applicationsxe2x80x9d, published in xe2x80x9cSensors and Actuators A 46-47xe2x80x9d (1995), pages 482-486.
When a light beam is guided in a waveguide, the associated electromagnetic field is not wholly restricted to the actual waveguide, but rather the field extends into the adjacent optically thinner areas (that is to say areas of a lower refractive index). This is referred to as a transversely attenuated wave. The field, which rapidly decreases in intensity as it becomes more distant from the waveguide, is referred to as an evanescent field. The depth of penetration into the medium of lower refractive index adjacent to the waveguide is in the order of magnitude of a fraction of a wavelength, which is comparable to the dimensions of relatively large biomolecules. Processes that take place outside the depth of penetration of the evanescent field are (essentially) not detected by the guided light and therefore have (essentially) no influence on a measuring signal generated as a result. That is the basis for the large number of uses of optical waveguides in sensory analysis. The proposed sensor platform is based on the change in the so-called effective refractive index in the region of the recognition layer, which is located at least partly in the evanescent field.
According to the invention, in the simplest case the planar sensor consists of a transducer and a recognition layer. The transducer again consists of a substrate (support material), optionally an intermediate layer, a waveguiding layer and optionally an adhesion-promoting layer for the immobilisation of the recognition layer. Specifically for thin waveguides where the thickness of the waveguiding layer is smaller than the wavelength of the light, the number of propagatable modes of the light field is limited to a few discrete waveguide modes.